The invention pertains to a process for preparing the walls of a mold for the molding or shaping of a molded part after completion of a molding cycle and removal of the molded part from the mold to make the mold ready for the next molding cycle, comprising the following steps:
(a) the mold walls are brought to the desired temperature; and
(b) a mold wall treatment agent is applied to the walls of the mold.
Processes of this type are known according to the state of the art and are used, for example, in the production of molded parts by molding processes such as those known in professional circles under names such as mold-casting, thixo-casting, thixo-forming, Vacural mold-casting, squeeze casting, etc. The state of the art will be explained below by way of example on the basis of the preparation of the mold walls of a mold for the die-casting of metal, but it is to be emphasized that analogous problems also occur in other shaping processes such as forging.
To produce a molded part, liquid or semi-liquid metal consisting of a light metal or heavy metal alloy is usually introduced under pressure into a divided, closed mold of steel and allowed to solidify. At the same time, the mold heats up as a result of the heat transferred to it from the solidifying material. Under production conditions, that is, during the production of as many castings as possible in the shortest possible time, the temperature of the mold would continue to increase. To achieve good-quality castings, however, the mold should have the same initial temperature at the start of each production cycle. Under production conditions, therefore, the mold must usually have heat removed from it continuously, so that thermal equilibrium is reached between the quantity of heat which the metal transfers to the mold and the quantity of heat which the mold releases as radiation to the surroundings or which is removed from it by supplemental cooling, with the result that an approximately uniform mold temperature is maintained.
Of course, instead of supplemental cooling, it may also be necessary to provide supplemental heating to the mold. This will be the case, for example, when only a small amount of metal is poured into a very heavy mold, that is, when molded parts with very thin members are produced. In this case, therefore, it can happen that the mold radiates off more heat to the surroundings that is desirable for the maintenance of a mold temperature favorable to the casting process. Therefore, with reference to the present invention, it is said in very general terms that the mold is xe2x80x9ctemperedxe2x80x9d, to cover both the possibility that the mold must be cooled as well as the possibility that it must be heated.
In addition to the need to temper the mold, it is also necessary to treat the surface of the mold walls with a lubricating and mold-release agent after removal of the last molded part and before the introduction of fresh liquid metal into the-mold. This mold wall treatment agent has the primary job of preventing the introduced metal from welding or sticking to the material of the mold, of guaranteeing that the finished part can be removed from the mold, and of lubricating the moving parts of the mold such as the ejectors or pushers. In certain processes, the mold wall treatment agent has the additional task of reducing the heat transfer between the introduced metal and the mold during the filling process. The layer of mold wall treatment agent applied to the mold wall should have the most uniform possible thickness, because the layer can rupture at points where it is too thin, and this will result in turn in the welding of the introduced metal to the mold material. If the layers are too thin, furthermore, too much heat can be transferred from the introduced metal to the mold, with the result that the introduced metal cools down too quickly just after it has been introduced and thus prevents the mold from being filled sufficiently. But layers which are too thick can also impair the quality of the castings by occupying too much of the volume of the mold.
According to the conventional method, the mold walls are sprayed with a mixture of mold wall treatment agent and water each time a molded part is removed from the mold, as described in, for example, DE 4,420,679 A1 and DE 195-11,272 A1. The advantage of the use of these mixtures of treatment agent and water is the savings in time, which results from the fact that the surface of the mold wall is cooled by the sprayed-on water at the same time that the mold wall treatment agent is applied to the walls. One of the problems which has had to be dealt with in this method, however, is the Leidenfrost effect. That is, when the droplets of spray land on the hot surface of the mold wall, a vapor barrier forms between the droplets and the surface. This barrier prevents the droplets from completely wetting the surface. Some of the sprayed-on mixture of treatment agent and water therefore runs off the surface of the mold wall without cooling it, lubricating, it, or wetting it, and giving it the required release properties.
To cool and the mold wall surface and to be able to coat it with mold wall treatment agent sufficiently in spite of this problem, it is necessary to apply an excess of the treatment agent-water mixture. But then the trade-off must be accepted that a considerable amount of the treatment agent-water mixture will run off the surface of the mold walls unused and then must be collected and disposed of. This raises significant problems in terms of environmental compatibility, which will be explained in greater detail below on the basis of an example.
If we assume that a foundry uses approximately 5 kg of mold wall treatment agent concentrate per 1,000 kg of cast aluminum and that this concentrate is diluted with water in a ratio of 1:100 before spraying, i.e., a total of about 500 liters of treatment agent-water mixture is sprayed, and if we also assume that about 80% of this amount runs off unused from the mold walls as excess, this means that approximately 400 liters of waste liquid must be disposed of per ton of cast aluminum. This is a conservative estimate. A less favorable but equally realistic estimate results in a volume of approximately 900 liters for disposal per ton of aluminum. In a medium-sized casting shop with a capacity of about 5,000 tons of aluminum per year, it is therefore necessary to dispose of 2,000-4,500 m3 of waste liquid.
Against this background it is the task of the present invention to improve the environmental compatibility of the process of the general type described above.
This task is accomplished in accordance with the invention in that, in the process of the general type in question, steps (a) and (b) are conducted in the sequence indicated, independently of each other. Thus, in step (a), the supply of heat to or the removal of heat from the mold walls is controlled as a function of the process conditions and/or the environmental conditions, preferably under the control of a program; whereas, in step (b), the mold wall treatment agent is applied in a controlled manner, preferably in a program-controlled manner. According to the invention, therefore, the mold walls, especially their surfaces, are first brought to the desired temperature before they are coated in a process independent of this tempering. Specifically, that is, there is no overlap in time between the tempering of the mold and the application of the mold wall treatment agent. The advantages of the process according to the invention will be explained in the following, again merely by way of example, on the basis of the use of the previously discussed casting process, in which the tempering of the mold walls usually takes the form of cooling.
As a result of the separation in time between tempering and coating, it is possible to allow each of the two component processes to proceed under the most favorable possible conditions for it alone, which has a favorable effect on the environmental compatibility of the process according to the invention.
First, the mold wall surface is cooled in a controlled manner under consideration of the process conditions and/or environmental conditions. This controlled cooling does not exclude the possibility that the coolant, preferably pure water, is applied in excess, at least in certain time intervals, to the mold walls to counter the Leidenfrost effect. As a result of cooling with an excess of water, a great deal of heat can be removed from the mold in a relatively short time, which makes it possible for the mold temperature desired for the next filling process to be approached quickly. During the final phase of the tempering process, however, the control of the cooling process makes it possible to adjust the temperature precisely to the desired value. Cooling with an excess is perfectly safe in terms of the environment, however, because water can be used as a coolant according to the invention, and the excess water running off the mold can be purified of metal and treatment agent residues by filtration, centrifuging, settling, sedimentation, etc., and then either reused or, under observance of the local regulations, easily discharged into the municipal sewer system.
Then the mold wall treatment agent is applied in a controlled manner. Because the mold walls have been cooled first, the degree to which the Leidenfrost effect interferes with the wetting of the mold wall surface is at least considerably less than it would have been according to the state of the art, if it occurs at all. To achieve a sufficient coating, therefore, the mold wall treatment agent does not need to be applied in an excessive quantity. At most, possibly only a very small excess will have to be applied to the mold wall surface, which means that either no disposal problems at all or correspondingly reduced disposal problems remain to be dealt with. The controlled application of the mold wall treatment agent makes it possible not only to minimize or to eliminate the excess but also to apply a uniformly thick layer of mold wall treatment agent to the mold wall surface regardless of the topography of the mold wall.
Because of the better environmental compatibility of the process according to the invention, the disposal costs associated with every molding process are correspondingly lower when the process is used, so that, in spite of the separation in time between the tempering and the coating of the mold wall, the economy of the process according to the invention is certainly no worse than that of the process according to the state of the art and possibly better overall. In addition, it should be noted that, through the controlled tempering and the controlled application of the mold wall treatment agent, it is possible to minimize the time required for a preparation cycle.
Another improvement in the environmental compatibility of the process according to the invention can be achieved by using ready-to-use mold wall treatment agent, for example, which is taken without dilution from a transport container and applied to the mold walls. By eliminating the step of diluting the mold wall treatment agent supplied by the manufacturer of the agent, various problems can be bypassed which until now have plagued the state of the art as a result of the need to dilute a mold wall treatment agent concentrate to a ready-to-use consistency. That is, water-diluted mixtures are susceptible to attack by bacteria or fungi, which can destroy the lubricating and mold-release properties of the mold wall treatment agent. Therefore, bactericides and the like must be added to the supplied mold wall treatment agent concentrate, and these agents for their part have a disadvantageous effect on the lubricating and mold-release properties of the mold wall treatment agent. In addition, the bactericides make it more difficult to dispose of the run-off excess in an environmentally safe manner.
Because, as proposed, the mold wall treatment agent is taken directly from the transport container and applied to the mold walls, i.e., is managed in a closed system, and also because the mold wall treatment agent is ready to use, the above-discussed dilution step is eliminated according to the invention, and the risk of attack by bacteria or fungi in the process according to the invention is minimized. This risk can be further reduced by keeping the transport containers carefully sealed, by using a removal device of appropriate design, and by similar measures. Thus it is possible to eliminate completely the use of bactericides. In addition, the personnel costs for the operation, maintenance, and monitoring of the mold wall treatment agent preparation and dilution system are also eliminated.
Corresponding logic applies to the use of the corrosion-proofing agents, which are added to water-diluted mixtures to protect the mold but which hinder the formation of a film of mold wall treatment agent on the mold wall surface. Because the agent according to the invention is not diluted with water, however, the addition of such corrosion-proofing agents can be reduced or even completely eliminated.
If an arrangement is used in which the mold spray system includes at least two transport containers, at least one of which is connected to a spray element to supply it with agent, whereas at least one other container is held in readiness for the same purpose, the advantage is obtained that, after the one transport container has become completely empty, it is possible to switch over either automatically or manually to the other transport container and to continue removing the agent from it. The production operation thus does not need to be interrupted; on the contrary, the empty container can be replaced with a new transport container filled with mold wall treatment agent as operations continue without a break.
If the mold wall treatment agent contains at least 98 wt. % of lubricating and mold-release substances (e.g., the mold wall treatment agent can contain at least one silicone oil or similar synthetic oil and/or at least one polyolefin wax such as a polyethylene wax or polypropylene wax as lubricating and mold-release substances) and no more than 2 wt. % of auxiliary materials such as corrosion-proofing agents, bactericides, emulsifiers, solvents such as water, etc., then it is possible to bypass another problem. Unless they are used immediately, water-diluted mold wall treatment agents tend to separate in spite of the addition of emulsifiers. This separation can be prevented by agitating the mixture, for example. Agitation, however, such as by means of mixing machines or centrifugal pumps, subjects the lubricating and mold-release substances of the mold wall treatment agent to repeated shear stress and impairs their lubricating and mold-release properties. Because of the absence of solvent, however, there is no need to fear separation, and it is therefore possible to eliminate the agitation of the mold wall treatment agent. This has a favorable effect on the lubricating and mold-release properties of the mold wall treatment agent, and at the same time it lowers the acquisition and maintenance costs of the system by eliminating the need for a mixing machine. Finally, it allows the effective utilization of the lubricating and mold-release substances.
Because of the small water content, furthermore, the application of the mold wall treatment agent to the hot mold wall surface is subject to little or no interference from the Leidenfrost effect. Therefore, the mold wall treatment agent, which can have a viscosity in the range of about 50-2,500 mPaxc2x7s at a temperature of 20xc2x0 C., for example (measured with a Brookfield viscometer at 20 rpm), can be brought into contact with a much hotter mold wall surface than was possible in the mold wall treatment systems explained above according to the state of the art. Thus, the mold wall surface does not need to be cooled down as much; this offers, first, the advantage of time savings and, second, the advantage of reduced thermal stress on the mold. Because the ready-to-use mold wall treatment agent is able to wet the mold walls and to form a lubricating and effective release layer on it even at a mold wall temperature of about 350-400xc2x0 C., the mold wall can be treated at a temperature favorable for the next molding cycle. These favorable temperatures are usually in the range of 150-350xc2x0 C., but they can also be even higher. Mold wall treatment agents with high-temperature wetting properties are described in, for example, U.S. Pat. No. 5,346,486.
The small water content of the mold wall treatment agent also offers the advantage that the layer applied to the mold wall surface also contains few if any water inclusions. In the presence of such water inclusions, there is the danger that the water vapor which forms from these water inclusions as the liquid metal is being poured into the mold cannot escape from the mold and leads to the formation of pores in the casting, which significantly impair its quality. This danger is significantly reduced if not completely eliminated when the water-free mold wall treatment agent according to the invention is used, with the result that castings with very few if any pores can be obtained.
With respect to the above-cited temperature range prevailing at the surface of the mold wall during the application of the mold wall treatment agent, it is proposed that the flash point of the mold wall treatment agent be at least 280xc2x0 C.
To ensure that the mold wall treatment agent is finely atomized, it is proposed that, for example, the mold wall treatment agent, in view of its composition and high viscosity as indicated above, be applied to the mold walls by means of at least one spray element with centrifugal atomization and air control . The design and function of spray elements such as this will be discussed in greater detail further below.
It should be emphasized, however, that the process according to the invention can also be implemented with conventional spray elements, especially when water-diluted mold wall treatment agents are used. For example, the spray elements known from DE 4,420,679 A1 and DE 195-11,272 A1 can be used.
As part of the controlled application of the mold wall treatment agent, the quantity of mold wall treatment agent discharged per unit time onto the mold walls can, for example, be detected by sensors, which measure the volume-flow rate and/or the mass flow rate. The thickness of the layer of mold wall treatment agent applied to the mold walls can be controlled by variation of the trajectory of the spray element, of which there is at least one, and/or by variation of the speed of spray element or elements and/or by variation of the quantity of mold wall treatment agent discharged per unit time by the spray element or elements.
As already mentioned above, when mold wall treatment agents without significant amounts of substances lacking lubricating or mold-release properties are used, and when the mold wall treatment agent is atomized finely in conjunction with program-controlled application which releases only very small amounts of gaseous components, thin, uniform layers of the mold wall treatment agent can be formed on the hot surface of the mold walls. This is especially important when the goal is to produce low-porosity or weldable castings.
Heat can be supplied to and removed from the mold walls in various ways. According to a first design variant, it is possible, for example, to apply an appropriately tempered fluid to the mold walls. In principle, the tempered fluid can be an appropriately tempered gas. Because of the better heat-transfer properties of liquids, however, the use of a tempered liquid such as water is preferred.
For example, the mold walls can be cooled by applying a liquid to, preferably by spraying a liquid onto, them and by allowing it to evaporate. According to an advantageous elaboration, demineralized water is used for this purpose, as a result of which a mold wall treatment agent layer highly effective in terms of its lubricating and release properties will be obtained. If, namely, as is conventional in the processes according to the state of the art, tap water is used, the CaO and MgO present in this tap water can, upon evaporation from the surface of mold wall, form a coating such as a lime deposit, which impairs the lubricating and release action of the mold wall treatment agent applied thereafter. In the worst case, this impairment can lead to the rupture of the mold wall treatment agent film as the metal is being poured in and thus to the welding of this metal to the mold. This can be prevented by the use of demineralized water. Although, in principle, it is possible to use additives which increase the tempering effect, according to what has been said above care should be taken to ensure that these additives do not interfere with the lubricating and release properties of the mold wall treatment agent. The corrosive effect of water, especially demineralized water, can be remedied by the addition of corrosion-proofing agents. The degree of demineralization and the amount of corrosion-proofing agent added can be selected under consideration of all the economic aspects.
As in the state of the art, the cooling liquid can be applied in excess to the mold walls, because, in the process according to the invention, the excess cooling liquid running down from the mold does not give rise to any environmental concerns. In addition, the cooling liquid running down from the mold walls can be collected and reused, possibly after a purification treatment such as filtration, centrifuging, settling, sedimentation, etc.
If necessary, the mold wall can be dried after it has been cooled with the liquid; it is preferably blown dry.
According to a second variant of the invention for arriving at the desired temperature at the surface of the mold walls, at least a certain area of the surface of the mold walls can be brought into contact with a heat-transfer device. It is understood that this contact tempering can also be used in addition to the fluid tempering discussed above. For example, contact tempering can be used to cool areas of the mold wall surface which are especially hot.
To achieve the best possible heat transfer between the mold wall surface and the heat-transfer device, it is proposed that the heat-transfer device comprise at least one heat-absorbing and/or heat-supplying body which is designed to fit the contours of the area of the mold wall to be tempered. The heat-absorbing and/or heat-supplying body or bodies can be mounted resiliently on a carrier and/or against one another, which facilitates the equalization of any thermal expansion or contraction of the heat-absorbing and/or heat-supplying bodies.
In a further elaboration of this alternative, it is proposed that the heat-transfer device be made at least partially of a good heat conductor such as copper, a copper alloy, aluminum, an aluminum alloy, etc., at least in the area of the heat-transfer surface.
To be able to supply heat to or to remove heat from the heat-transfer device while it is in contact with the mold wall surface, it is proposed that the heat-transfer device for removing or supplying heat be connected to a heating-cooling machine. In addition or as an alternative, however, it is also possible for the heat-transfer device to be immersed in a heating-cooling bath to supply heat to it or to remove heat from it in preparation for heat-transferring contact.
To produce the heat-transferring contact between the heat-transfer device and the mold wall, the mold can be at least partially closed. The heat-transfer device can be moved into the mold by an industrial robot known in and of itself, preferably a six-axis robot, brought into contact with the mold, and then pulled back out of it again.
Another design variant for supplying heat to or removing heat from the mold is to connect the mold directly to a heating-cooling machine, which allows heat-transfer fluid to flow through a system of channels in the mold.
The temperature of the mold wall can be detected as a possible input variable for the controlled tempering of the mold wall surface. One way in which this can be done is to install a temperature sensor on at least one site which is representative of the temperature distribution of the mold wall and/or which is especially critical in terms of temperature. In addition or as an alternative, the temperature of the mold wall surface can also be measured by means of an infrared measuring device, which supplies digital and spatially resolved thermal images of the mold wall surface which are both time-resolved and also near-instantaneous. If a direct determination of the temperature distribution of the mold wall surface by means of the infrared measuring device is not possible, the distribution can be deduced indirectly by analysis of the thermal images of a molded part just released from the mold. Temperature-critical sites of the molded part can also be brought into contact with a temperature sensor.
The above-described indirect determination of the temperature distribution of the mold wall surface by measurements of a just-finished molded part has the advantage that the infrared measuring device or the temperature sensor can be mounted permanently at a site adjacent to the mold, which means that there is no longer any need for a robot arm to move this measuring device or sensor or in particular to introduce this measuring device into the mold.
Especially when the infrared measuring device discussed above is used, the temperature at a predetermined location on the surface of the mold wall can be detected a predetermined length of time after the opening of the mold and the removal of the molded part. The temperatures specific to time and place thus obtained in successive molding and mold wall treatment cycles can then be compared with each other. In this way it becomes possible to draw conclusions concerning the stability of the overall molding and mold wall treatment operation and to intervene with corrective measures as necessary. For example, if it has been found that the temperature at a predetermined point in time and space is increasing from cycle to cycle, the intensity of the cooling of the mold wall surface can be increased accordingly. If a temperature exceeds a predefined value, it is possible to conclude that there is a defect in the tempering device, and the entire molding process can be stopped to prevent the production of rejects and to avert damage to the mold. A similar type of decision can also be made when the above-discussed volume-rate of flow and/or mass-rate of flow sensor detects that too little mold wall treatment agent is being dispensed.
In addition, the heat balance control strategy explained above can also take into account the ambient temperature, because the outside temperature prevailing at the location of the mold also affects the intensity of the thermal radiation from the mold. The ambient temperature, however, changes with the seasons, for example, and also as a result of changes in exposure to sunlight.
In addition, the course of the working or production procedure should also be taken into account, because there is the danger that the mold could cool off too much while the system is idle, in which case the temperature of the mold wall surface would fall below the desired value. The same is also true during the startup of the mold wall treatment system at the beginning of the work day.
When fluid tempering is used, the supply of heat to or removal of heat from the mold wall can be controlled by adjusting the quantity of fluid supplied per unit time to the mold wall and/or by adjusting the duration of this application. When contact tempering is used, the supply of heat to or the removal of heat from the mold wall can be controlled by adjusting the duration of the heat-transferring contact between the mold wall and the heat-transfer device and/or by adjusting the initial temperature of the heat-transfer device.
The spray elementxe2x80x94at least one of which is providedxe2x80x94with centrifugal atomization and air control, which was mentioned briefly above and which will be explained in greater detail further below, can be mounted on a spray tool which introduces it into the mold. When the mold wall surface is tempered with fluid, furthermore, at least one discharge element for dispensing the tempering fluid can also be mounted on this spray tool. In addition, at least one discharge element for dispensing blown air can also be mounted on the spray tool; this air can be used, for example, to clean the mold of treatment agent residues or to blow-dry the mold. Finally, the spray tool can be moved by the arm of a preferably six-axis robot, preferably a program-controlled robot. This has the advantage that the spray tool is highly mobile and can spray every point on the mold wall from a suitable point along its trajectory and with a suitable orientation, so that even mold areas with complicated contours such as undercuts and recessed areas can be coated with the desired uniformity.
From another viewpoint, the invention pertains to a device for preparing the walls of a mold for the molding or shaping of a molded part after completion of the molding cycle and removal of the molded part from the mold to prepare the walls of the mold for the next molding cycle. With respect to the design and function of this mold wall treatment device and the advantages which can be achieved by its use, reference is made to the discussion of the process according to the invention discussed above.
According to yet another viewpoint, the invention pertains to a spray element for spraying the walls of a mold for the molding or shaping of a molded part with a mold wall treatment agent, the spray element comprising a rotor, which is mounted in a spray element body so that it can rotate around an axis, to one longitudinal end of which rotor an atomizing element is attached, the spray element also comprising a feed line for mold wall treatment agent, from which the mold wall treatment agent is able to pass to the atomizing element, and a feed line for control air, which serves to direct the mold wall treatment agent atomized by the atomizing element to the mold wall to be sprayed, and where an outlet of the control air feed line is provided near the outside periphery of the atomizing element. That is, the invention also pertains to a spray element with centrifugal atomization and air control as has already been mentioned several times above.
Spray elements with centrifugal atomization and electrostatic control are known from coating technology. Reference can be made merely by way of example to DE 4,105,116 A1, DE 2,804,633 C2, and EP 0,037,645 B1. In this spray technology, high voltage is applied to the spray element during the coating process, whereas the body to be coated is, for example, grounded. The paint supplied to the rotating atomizing element is atomized by the action of centrifugal force, and the fine paint droplets are electrostatically charged simultaneously. Although the paint droplets are flung away by the atomizing element at right angles to the axis of the rotor, the fact that they are charged means that they follow the field lines of the electric field between the spray element and the body to be coated and thus arrive on the surface to be painted. The above-described spray elements with centrifugal atomization and electrostatic control cannot be considered for spraying the walls of a mold for molding or shaping, because the cost of the equipment and of the safety systems required for the use of electrostatic control is so high that it would make the molding or shaping process as a whole uneconomical. In addition, the Faraday effect interferes with the spraying of concave surface areas of the mold wall surface, especially holes, ribs, gaps, etc., such as those which are frequently found in molds for castings such as engine blocks, crankshafts, etc.
It must also be remembered that the spray element is intended to apply essentially solvent-free mold wall treatment agents such as those considered above for the spraying of mold wall surfaces in an accurately measured, finely distributed, and uniform manner onto the mold wall surface. As already mentioned, essentially solvent-free mold wall treatment agents of this type, that is, mold wall treatment agents which contain at least 98 wt. % of substances with lubricating and release properties and no more than 2 wt. % of auxiliary materials such as bactericides, emulsifiers, solvents such as water, etc., usually have a viscosity in the range of about 50-2,500 mPaxc2x7s (Brookfield viscometer, 20 rpm) at a temperature of 20xc2x0 C. and are applied in a quantity much smaller than that used according to the state of the art to the mold wall surface. It must be remembered that the concentrates delivered by producers of mold wall treatment agents usually contain only about 5-40 wt. % of substances with lubricating and release properties and is diluted even further before use in a ratio of 1:40-1:200. With the spray element according to the invention, therefore, the volume sprayed per unit time is about 1,000 times smaller than that of the conventional spray elements.
The task of the invention, however, is to provide a spray element for coating the walls of a mold for molding or shaping between two successive molding cycles, that is, a spray element which is able to apply to the mold wall surface even an essentially solvent-free, viscous mold wall treatment agent in a layer thickness suitable for the next molding cycle, this being accomplished under simultaneous preservation of the economic benefit of the molding process.
In spite of the small mold wall treatment agent throughput, the centrifugal atomization used by the spray element according to the invention is able to atomize the agent with the required uniformity over time in a precisely measured fashion. The atomized mold wall treatment agent is then taken up by the control air and deflected from the direction in which it is being propelled, namely, at a right angle to the axis of the rotor, in such a way that it moves essentially in the main spray direction, that is, in the direction of an extension of the rotor axis, toward the mold wall surface. The use of compressed air to guide the mold wall treatment agent spray mist has the advantage that this is usually already available in systems for molding or shaping and thus does not require any additional investment. This aspect is also of interest in terms of the retrofitting of already existing spray systems with the spray elements according to the invention. In addition, compressed air is a relatively safe medium, with which machine operators and maintenance personnel have long been familiar.
It must be kept in mind, however, that the spray element according to the invention is also suitable for spraying water-diluted mold wall treatment agents and water. The adaptation to the lower viscosity of these materials can be accomplished by, for example, an appropriate choice of the rpm""s of the atomizing element and by appropriate adjustment of the control air throughput.
To be able to ensure that the mold wall treatment agent spray mist leaving the atomizing element is entrained as completely as possible by the control air, the outlet of the control air feed line can, in accordance with a first alternative design variant, comprise a plurality of outlet openings arranged in a circle around the atomizing element. According to a second alternative design variant, the outlet of the control air feed line can comprise an outlet slot forming a circle surrounding the atomizing element. To be able to ensure that the pressure of the control air is as uniform as possible in the circumferential direction, it is proposed that the control air feed line include a ring-shape channel upstream of the outlet slot.
To adjust the included angle of the spray cone, it can be provided, for example, that the control air feed line is formed at least in part by a head part of the spray element body, which is movable relative to a base part of the spray element body, such as by means of a preferably program-controlled servo drive. The boundaries of the ring-shaped channel can be formed on the radially outward side by the head part and on the radially inward side by the base part or by an element connected to the base part.
So that the control air can be ejected in a jet-like, controlled manner, the control air feed line can be designed with a taper near the outlet end, tapering down in the outlet direction of the control air.
A drive unit for producing the rotational movement of the rotor around its axis of rotation can comprise, for example, a turbine operated with compressed air, which represents a low-cost design variant, because compressed air is being supplied in any case to the spray element as control air. Alternatively, the drive unit can also be an electric motor or some other suitable type of rotary drive. The drive unit can be mounted in a housing which is separate from the base of the spray element body and which can be attached to the base. This facilitates accessibility for maintenance, for example.
The atomizing element can form a single unit with the rotor, or it can be connected detachably to it by means of, for example, quick-release devices.
In accordance with a first alternative design variant, it can be provided that the atomizing element has an atomizing surface facing the mold wall surface. It is advantageous for the atomizing surface to extend radially outward and away from the spray element in the direction of rotation, in such a way that the atomizing surface forms a cone, where half the included angle of the cone is, for example, between about 30xc2x0 and 60xc2x0, preferably about 45xc2x0. An atomizing surface with this design is advantageous, because the mold wall treatment agent is thus pressed by the centrifugal forces acting on it against the atomizing surface and can be effectively atomized by it under the effects of friction. The atomizing element can thus, for example, have an atomizing funnel opening in the direction of the mold wall surface, the inside surface of the funnel acting as the atomizing surface.
So that the mold wall treatment agent can be discharged in the most uniform possible manner onto the atomizing surface, it is proposed that the atomizing surface be preceded by a distribution chamber. This distribution chamber can have an opening near the axis of rotation and extending around the axis of rotation, through which mold wall treatment agent is introduced; and a distribution chamber boundary surface, which extends radially outward, pointing away from the direction [plane?xe2x80x94Tr. Ed.] of rotation, can adjoin the outer circumferential edge of the opening. The distribution chamber boundary surface can be conical, for example, where half the included angle of the cone can be, for example, between about 20xc2x0 and about 60xc2x0, preferably about 45xc2x0.
The mold wall treatment agent introduced into the distribution chamber through the radially inward opening is forced radially outward by the centrifugal forces acting on it in the chamber; the boundary surface of the distribution chamber prevents the re-emergence of the mold wall treatment agent from the distribution chamber and thus protects the spray element from contamination. Distribution passages, which lead from the distribution chamber to the atomizing surface, can be provided in the area of this radially outward holding space, which is at least partially defined by the distribution chamber boundary surface, that is, in the peripheral area of the distribution chamber remote from the axis of rotation. These distribution passages can be simple holes or slots to minimize the cost of fabricating the atomizing element. In terms of production technology, it is also favorable for these holes or slots to extend in the radial direction. In principle, however, it is also conceivable that the holes or slots could be at a predetermined angle to the radial direction. By the use of appropriate methods to fabricate the atomizing element, the distribution passages can also be curved, so that an effect comparable to that of guide vanes is obtained.
If the outer peripheral edge of an element forming the boundary between the distribution chamber and the mold wall projects in the radial direction beyond the radially outer edge of the distribution passages and is mounted a certain distance away from the atomizing surface, it is possible to offer the distribution passages a certain protection from damage. In addition, the atomizing element as a whole obtains an attractive outer appearance.
In particular, however, the gap present in the above design between the atomizing surface and the element forming the boundary between the distribution chamber and the mold wall has another advantageous effect. If the atomizing element is running empty, that is, without any mold wall treatment agent being supplied to it, the air enclosed in this gap is propelled radially outward by centrifugal force so that a negative pressure, which draws air out from the distribution chamber, is created in the area of the outlet of the distribution passages. Overall, therefore, what develops is a blower-like effect, which ultimately leads to the self-cleaning of the atomizing element after the coating of the mold wall surface has been completed.
After the mold wall treatment agent has been introduced into the distribution chamber, its movement into the distribution passages can be facilitated by providing a rounded transition from the cylindrical boundary surface of the distribution chamber, which is essentially coaxial to the axis of rotation, to the boundary surface of the distribution chamber, which extends essentially at a right angle to the axis of rotation. This is important especially as a way of ensuring the completeness of the above-mentioned self-cleaning of the atomizing element.
The atomizing element according to the first alternative design variant of the invention discussed above can be designed as a single piece or as several pieces. In the latter case, the individual parts of the atomizer element can be joined together by pressing, flanging, or the like.
In accordance with a second alternative design variant, the atomizing element can comprise an atomizing disk.
So that maximum advantage of the centrifugal effect of the atomizing element can be taken, it is proposed that the mold wall treatment agent emerging from the mold wall treatment agent feed lines strike the atomizing element near its axis of rotation.
If the spray element comprises a plurality of mold wall treatment agent feed lines, the area of the mold wall which require special treatment can be coated separately with one or more mold wall treatment agents. It is also possible, however, to coat the entire mold wall treatment agent with a multi-layer coating of various mold wall treatment agents. Mixed layers can also be applied by the simultaneous discharge of mold wall treatment agent from at least two of the mold wall treatment agent feed lines.
For the spraying of concave mold wall sections such as holes as well as ribs and gaps, it can be advantageous to provide a device for deflecting the main discharge direction of the spray element out of the extension of the axis of rotation of the rotor. There are many different design variants which could be used to realize such a deflecting device. For example, the deflecting device can be a device for changing the number and/or diameter of outlet openings and consist, for example, of a diaphragm ring. As an alternative, however, it is also possible for the deflecting device to be a device for changing the width of the outlet slot, and consist again, for example, of a diaphragm ring. But it is also possible to provide a plurality of control air feed lines, the air throughput of which can be adjusted independently of each other. In this case, the deflecting effect is achieved by appropriate adjustment of the air throughput through the majority of feed lines to different values. Finally, it is also possible for the deflecting device to consist of at least one deflecting air feed line; that is, an additional deflecting air feed line is provided, which is xe2x80x9cturned onxe2x80x9d as needed.
As a further elaboration of the invention, it is provided that the thickness of the layer of mold wall treatment agent applied to the mold walls can be controlled, preferably in a program-controlled manner. The thickness of the applied layer can, for example, by controlled by adjusting the speed at which the spray element travels and/or by adjusting the quantity of mold wall treatment agent discharged per unit time by at least one spray element.
From a different viewpoint, the invention pertains to the use of a spray element according to the invention as part of, if desired, a mold spray device according to the invention and also, if desired, within the scope of the implementation of the above-described mold wall treatment process according to the invention for spraying the walls of a mold for molding or shaping with an essentially solvent-free mold wall treatment agent. The advantages of this use can be derived from the discussion given above.
The invention is explained in greater detail below on the basis of the attached drawing: